This project is concerned with further exploration of an experimental model for detecting and measuring in vitro the elements of specific recognition and responses by lymphoid cells of the tumor-bearing host to solubilized tumor-specific antigens. MCA-induced fibrosarcomas in various inbred mouse lines provide the tumor-bearing host material for the study, and KCl-solubilized tumor membranes from the tumor provide the solubilized tumor antigen for in vitro stimulation tests. The model will be further characterized in terms of conditions for eliciting stimulatory, as contrasted with inhibitory, responses in various cell subpopulations. The cross-reactions of soluble MCA-TSA in vitro, in contrast to noncross-reacting TSTA found in transplantation tests, will be explored further. Differential contributions of T and B subclasses of lymphocytes to the stimulatory/inhibitory effects will be noted. Quantitative approaches to enumeration of the various cell elements will be applied. A second major area of investigation involves attempts to understand the mechanism of inhibition or blocking of the various responsive cell subpopulations by tumor antigen fragments or fragment-antibody complexes. A modulation hypothesis is offered for testing of various predictions, using cell populations from tumor-bearing hosts and the solubilized tumor antigens as reagents.